1. Technical Field
The present disclosure relates to a bus system for a semiconductor circuit.
2. Description of the Related Art
Portion (a) of FIG. 1 illustrates an example of a lumped bus control. In a conventional integrated circuit which performs such a lumped bus control, a number of bus masters and a memory are connected together with a single bus, and accesses to the memory by the respective bus masters are arbitrated by an arbiter. However, as the functionality of an integrated circuit has been further improved and as the number of cores in an integrated circuit has been further increased these days, the scale of the circuit has become even larger and the traffic flows running through the bus has gotten even more complicated. As a result, it has become increasingly difficult to design an integrated circuit by such a lumped bus control.
Meanwhile, semiconductor integrated circuits with distributed buses have been developed one after another lately by introducing parallel computerized connection technologies and network control technologies such as ATM (asynchronous transfer mode). Portion (b) of FIG. 1 illustrates an example of a distributed bus control. In a semiconductor integrated circuit with distributed buses, a number of routers are connected together with multiple buses. Recently, people have been working on a so-called “Network on Chip (NoC)” in which the traffic flows in a large-scale integrated circuit are transmitted through a number of buses by adopting the distributed buses such as the ones shown in Portion (b) of FIG. 1.
However, even on an NoC which performs such a distributed bus control, the traffic flows running through the buses tend to increase so much that there is a growing demand for improved bus transmission performance. To maintain sufficient bus transmission performance, a method for speeding up the transmission by using a number of parallel transmission lines as the traffic flow increases may be adopted.
Japanese Laid-Open Patent Publication No. 2009-49742 discloses a general communications system in which a plurality of communications terminals communicate with each other over a network compliant with the Ethernet™ standard while sharing the same transmission lines with each other. As a method for speeding up the transmission using the transmission lines in parallel, Japanese Laid-Open Patent Publication No. 2009-49742 proposes a communications scheme for realizing broadband communications by making a single transmission terminal use multiple transmission lines.
FIG. 2 illustrates a configuration for the communications system disclosed in Japanese Laid-Open Patent Publication No. 2009-49742. In this communications system, transmission terminals illustrated on the left-hand side of transmission lines and reception terminals illustrated on the right-hand side of the transmission lines communicate with each other at higher speeds through a plurality of transmission lines.
According to conventional technologies, the communications quality of each transmission line is measured as a round trip time (RTT). If the RTT is long, the decision is made that the transmission line is congested, and therefore, the transmission rate is lowered. On the other hand, if the RTT is short, the decision is made that the transmission line is uncongested, and therefore, the transmission rate is raised. That is why since transmission and congestion control are supposed to be carried out continuously according to the conventional technologies until the transmission lines are too congested to avoid a breakdown, it is difficult to transmit data while maintaining expected transmission quality (see Japanese Laid-Open Patent Publication No. 2009-49742 and Evaluation of Fast Reliable Transport Protocol on Multiple Paths, IEICE Technical Report, February, 2005).
In a general network which uses the Ethernet™, for example, if any breakdown such as packet overflow happens at a buffer in a router, the router will discard those packets automatically. On the other hand, unlike the general network, no routers in an NoC will discard packets. The reason is that in an NoC, it is determined in advance what bus masters are to be connected to the network and it is possible to predict how much traffic flow data will run through the network, and therefore, the NoC can be designed so as to avoid discarding packets.
That is why the techniques that have been applied to the general network cannot be applied as they are to an NoC. It is difficult to speed up the transmission by using a number of transmission lines in parallel by the conventional technologies in order to maintain sufficient transmission performance for the bus.